Coal coming out of mines is required to be crushed first in a crushing device at power plant yards. The Crushing device crushes coal with around 2-centimeter diameter. This crushed coal is then pulverized to powder in coal mills. This pulverization is required for better burning in a boiler. Burning of coal releases heat to produce steam at the boiler. This steam rotates a turbine and alternator to produce electricity.
Pulverized coal coming out from coal mill is required to be evenly distributed in all individual burners for better burning and desired boiler efficiency. Otherwise it results in unburnt coal and unequal temperature at different zones of the boiler. In a typical boiler, coal particle and a primary airflow are fed from a pulverizer to the burners through a network of fuel lines. A single large diameter main conduit along with further branching of numerous small diameter conduits form a fuel line. Primary air helps the pulverized fuel to move towards the boiler. A single conduit is used in carrying pulverized fuel from the coal mill and further branches into smaller conduits to feed burners of the boiler. At branching of single conduit into numerous smaller conduits a flow imbalance takes place. Many users complain about the phenomenon of unequal temperatures in-side the boiler. Investigations revealed that unequal flow after the coal mill is a major cause of such imbalance.
Coal has to travel a long path sometimes about 30 to 50 meters after outlet at coal mill. Because of differences in conduits lengths and numbers and types of elbows in each fuel line, the different conduits from a pulverize mill usually have different flow resistances.
Sometimes an orifice is fitted in each fuel line coming out of the mill. Orifices help to reduce flow imbalance only. But these orifices or flow restrictors are not sufficient. The long travel increases possibility of unequal flow through conduits.
Another phenomenon takes place with high velocity air (around 30 meter per second) is the formation of concentrated flow in one side of large diameter conduit, which is called “rope formation”.
Imbalances in pulverized fuel flow through conduits also lead to maintenance problems associated with conduits erosion and/or clogging for example excessive localized coal accumulation, damage to burners and wind boxes, and accelerated water wall wastage. Problems such as these reduce the operating flexibility of the boiler.
Effort of equal flow to each burner thus gets impaired due to complexity of the situation. A burner imbalance leads to higher carbon monoxide emissions and high levels of unburned carbons. Large and old boilers thus suffer from lesser effectiveness of burners. Most of the boilers have numerous burners that are to be fed by a single coal mill.
Also lesser space to accommodate new device is a typical problem in existing boilers. Currently there is a need for a solution in industry for removal of imbalance in coal flow to burners. At the same time the boilers require homogeneous flow of pulverized fuel (crushed coal). The solution needs to be used in space constraints and it should be cost effective.
The problem arises when coal mill outlet branches out in several numbers of smaller diameter conduits or ducts to reach individual burner of a boiler. The resistance of each branches being unequal due to different lengths and varieties of configuration causes unequal output of pulverized fuel. Concentrated flow of pulverized fuel (coal rope) inside the conduit is another problem that causes erosion and unequal flow. Although here we described the problems due to coal, similar problems are also there when any other fuel is used.